In the late 1940's a transition in the manufacture of silver halide photographic products began away from the use of single jet emulsions toward to the use of double jet emulsions. The disadvantage of single jet emulsions was that they were polydisperse. They contained a wide range of grain shapes and sizes. This was the direct result of running silver salts into a halide salt solution of fixed volume in the reaction vessel and thereby varying the ratio of silver to halide continuously throughout the emulsion make.
In double jet precipitation both the halide and silver salts are concurrently introduced into the reaction vessel. Thus, it is possible to produce a grain population of little or no variance in grain shape and a very narrow distribution of grain sizes. Silver halide emulsions having a low variance of grain sizes are referred to as monodisperse emulsions.
Monodisperse emulsions are recognized to offer a variety of photographic advantages. For example, a larger percentage of the grains in a monodisperse emulsion can be optimally sensitized as a result of their similar surface areas. Fine grain populations, which disproportionately contribute to light scattering and therefore image sharpness reduction, are restricted. Larger grain populations, which contribute disproportionately to image granularity, are restricted. The reproducibility of the emulsions and their photographic performance rises as dispersity is reduced. Contrast of a single monodisperse emulsion is higher than that of polydisperse emulsion of the same mean grain size. Monodisperse emulsions are employed not only for photographic applications requiring higher contrast, but are also blended to achieve aim contrasts in photographic applications requiring relatively lower contrast, since a blended monodisperse emulsion retains photographic advantages over a polydisperse emulsion of the same mean grain size and contrast.
Maternaghan U.S. Pat. Nos. 4,150,994 and 4,184,878 are representative of early reported attempts to prepare tabular grain silver bromoiodide emulsions. Covering power advantages were postulated. Low coefficients of variation were reported for the emulsions. However, in retrospect this is not surprising, since from remakes and grain characterizations the average aspect ratios (most simply measured as mean grain diameter divided by mean grain thickness) of these emulsions are approximately 4:1.
Subsequent to Maternaghan, intensive investigations of high aspect ratio silver bromoioide emulsions were reported as well as procedures for their preparation. The average tabular grain aspect ratios of these emulsions were in all instances greater than 8:1. Wilgus et al U.S. Pat. No. 4,434,226, Kofron et al U.S. Pat. No. 4,439,520, Solberg et al U.S. Pat. No. 4,433,048, Daubendiek et al U.S. Pat. No. 4,414,310, Jones et al U.S. Pat. No. 4,478,929, Evans et al U.S. Pat. No. 4,504,570, and Maskasky U.S. Pat. No. 4,435,501 are representative of the earliest published teachings relating to high aspect ratio silver bromoiodide emulsions. More recently Daubendiek et al U.S. Pat. Nos. 4,693,964 and 4,672,027 have reported the preparation of high aspect ratio silver bromoiodide emulsions of much smaller mean grain diameters, referred to as small, thin tabular grain silver bromoiodide emulsions. Maskasky U.S. Pat. No. 4,713,320 illustrates the effect of gelatin methionine reduction on silver bromoiodide high aspect ratio tabular grain emulsion preparation.
The advantages of silver bromoiodide high aspect ratio tabular grain emulsions include an improved relationship between speed and granularity, sharper images--both in single and multilayer photographic elements, accelerated development, higher insensitivity to temperature variations during development, higher fixing rates, more favorable toning, higher covering power, an increased separation between minus blue (green or red) and blue speeds when spectrally sensitized to the minus blue portion of the spectrum, increased blue speed when spectrally sensitized to blue light, and a variety of other advantages observed in the context of specific photographic applications.
Although almost all silver bromoiodide high aspect ratio tabular grain emulsions are prepared by double jet precipitation techniques, difficulties were experienced from the outset in reducing the dispersity of the emulsions. Whereas regular grain emulsions produced by double jet precipitation (e.g., regular cubic or octahedral grain emulsions) can be readily prepared containing only the desired grain population, tabular grain emulsions are rarely prepared with only tabular grains present. Thus, having mixed populations of tabular and nontabular grains is one source of dispersity in tabular grain emulsions. The second source of dispersity is the dispersity variances within the tabular grain population itself, which is a function of the twinning followed by edge deposition growth pattern that distinguishes tabular grain emulsions from regular grain emulsions, wherein twinning is absent or rare and deposition favors no particular set of crystal faces. Further, dispersity in tabular grain emulsions increases as the average aspect ratios of the tabular grains increases. Therefore, dispersity levels which are easily attained in lower aspect ratio tabular grain emulsions have not been attainable at higher aspect ratios.
Himmelwright U.S. Pat. No. 4,477,565 and Sugimoto et al U.S. Pat. Nos. 4,609,621, 4,656,120, and 4,665,012 are illustrative of follow-on disclosures of variations in the preparation of high aspect ratio tabular grain silver bromoiodide emulsions.
It has been recognized from the outset of high aspect ratio tabular grain emulsion investigations that silver bromide tabular grain emulsions are much more readily prepared to exhibit both high aspect ratios and low levels of dispersity than corresponding silver bromoiodide emulsions. Research Disclosure Vol. 232, Aug. 10, 1983, Item 23212, (Mignot French Patent 2,534,036 corresponding) produced by a ripening procedure tabular grain silver bromide emulsions of average aspect ratios of 10, 12, and 25.6 with coefficients of variation of 15, 16, and 28.4, respectively. Research Disclosure and its predecessor Product Licensing Index are publications of Kenneth Mason Publications, Ltd., Emsworth, Hampshire P010 7DD, England. Saitou et al West German OLS 3,707,135 Al employs double and single jet precipitation techniques to produce silver bromide emulsions which exhibit higher coefficients of variation at aspect ratios comparable to those of Mignot, even though Saitou et al reports coefficients of variations based solely on the tabular grain population.
The prior processes while producing suitable emulsions for photographic uses nevertheless could be improved by formation of more monodisperse emulsions that have a higher coefficient of variation for thin tabular grains having an aspect ratio of greater than 8.